1. Field of the Invention
The present invention relates to the field of bandpass filters, and more particularly, to bandpass filters used to capture a wobble signal utilized in the read head and write head circuits of optical disk apparatus.
2. Description of the Related Art
Bandpass filters are commonly used in Digital Versatile Disk (DVD-RAM) and Compact Disk (CD-RW) apparatus. Specifically, bandpass filters are used in the read channel and write circuits of these devices. In operation, the bandpass filter serves to capture the wobble signal used in the positional control of the read and write heads associated with the optical disk apparatus. The wobble signal is typically embedded in a group of sinusoidal signals transmitted to the heads of an optical disk apparatus, with the frequency of the signal typically being in the range of 80-200 Khz.
Capture, which is the selective passing of the wobble signal through the bandpass filter, is a function of the center frequency of the bandpass filter. The more accurate the determination of the center frequency of the bandpass filter, the greater the accuracy of the filter in passing only the frequency associated with the wobble signal without passing additional or extraneous signal frequencies unassociated with the wobble signal. Accordingly, it is desirable to have an efficient and accurate means to determine the center frequency of a bandpass filter in order to optimize the capture of a wobble signal.
Conventionally, measurement of the center frequency of a bandpass filter is accomplished by measuring the maximum of the magnitude characteristic of the filter in question. However, bandpass filters generally have relatively flat magnitude characteristics about the center frequency, which makes difficult accurate measurement or test of the center frequency within, for example, an accuracy of 1%, since there is such a small differential in the magnitude of the characteristic of a bandpass filter about the center frequency.
Conventional devices have been proposed such as those disclosed in Whitten (U.S. Pat. No. 3,643,173) and Cabot (U.S. Pat. No. 5,136,267). These conventional devices incorporate tuneable microelectronic bandpass filters to achieve an effective inductance--and, hence, tuning of the center frequency--over a band of frequencies. To the inventor's knowledge however, none of the prior art devices presents a method or apparatus for measuring the center frequency to the degree of accuracy afforded by the present invention.